1. Field of the Invention
The present invention relates to an earthquake-resistant load-bearing system, and relates particularly to a system utilizing an isolated bearing for supporting a heavy structure such as a bridge, or a building or the like. It more particularly relates to an isolated bearing mounted on a relatively narrow surface such as a bridge-supporting pier or the like, and supporting an upper structure such as a bridge main body, for example. More particularly, the invention relates to an isolated load-bearing system comprising an upper bearing and a lower bearing supporting an upper construction load upon a lower base wherein the upper and lower bearings positioned in slidable contact with each other.
2. Description of the Related Art
In constructing a pier for a bridge, for example, a structure is made such that a bearing is placed between the upper surface of the pier and the lower structure of the upper part of the bridge, thereby supporting the load of the upper structure upon the pier. If the bearing is a conventional one, the whole pier and the whole bridge are in danger of receiving damage if it encounters a major natural disaster such as an earthquake, for example. In recent years, many suggestions have been made for protecting structures from earthquakes.
Among them, one suggestion embodies using laminated rubber in all or a part of the intermediate bearing construction. However, this encounters a problem of rocking, and is not suitable.
In Examined Published Japanese Patent No. 4-65193, there is disclosed a technique in which the position of an upper structure, which has temporarily moved due to an earthquake, is restored to its original position by gravity. However, in accordance with this gravity-oriented technique, the supporting surface requires a significantly widened area since the apparatus used for preventing the structure from inverting, by vertical oscillation due to the earthquake, is independently provided out of the isolated bearing main body. This is not suitable for use in a structure which is mounted on a supporting surface of limited area, such as a pier only, which sustains the upper structure, and which has a relatively narrow area.
On the contrary, as an example of an isolated bearing being made compact, mention is made of a buffer-type spherical bearing as disclosed in Examined Published Japanese Utility Model No. 7-56326. This is a spherical bearing having a spherical seat between an upper bearing and a lower bearing, and is provided with buffer members opposing each other in a sliding direction. It has the purpose of preventing motion due to an earthquake. An unevenness is provided in a center portion of a spherical contact portion between the upper bearing and the lower bearing, and the structure is made such that a small amount of movement is absorbed by the spherical seat. The isolated bearing is compact, but is hardly useful for absorbing the vertical oscillation of an earthquake. Further, since the structure is designed around a spring that absorbs vibration energy by horizontally moving with respect to the horizontal oscillation, the system is not suitable for restricting responsive displacement with respect to strong vibration. Still further, the periphery of the contact portion between the upper bearing and the lower bearing is directly exposed to wind and rain, and dust can easily enter between.
For example, in a heavy bridge, a collision occurs when a great amount of responsive displacement of the upper structure occurs, it is necessary to restrict the responsive displacement at the time of the earthquake to a limited degree, and to damp out the resulting oscillation as soon as possible. Further, since an isolated bearing mounted on a pier is directly exposed to wind and rain, and since dust can easily enter into the bearing portion, it is necessary to overcome this problem as well. However, there has been no concrete suggestion so far.